The Biden administration is working with European allies to expedite more sophisticated air-defense systems and other armaments into the Ukrainian war zone, U.S. officials said Friday.

Discussions were ongoing ahead of Defense Secretary Lloyd Austin’s planned trip this week to meet with NATO allies in Brussels and Slovakia, which along with Poland and Romania has indicated a willingness to transfer military aid to its embattled neighbor. Slovakia also possesses the S-300 surface-to-air missile system, which is used to shoot down enemy aircraft and is familiar to the Ukrainians.

Pentagon spokesman John Kirby told reporters that the United States is committed to arming the government in Kyiv with “the kinds of capabilities that we know the Ukrainians need and are using very well.”

“Some of that material we have and are providing. Some of that material we don’t have but we know others have, and we’re helping coordinate that as well,” Kirby said.

The administration is facing backlash over its decision last week to scuttle Poland’s proposal that would have sent a number of its MiG-29 fighter jets to Ukraine via a transfer “free of charge” to the United States. Washington, citing concerns that Russia would view the move as a provocation from Washington, said the offer from Warsaw was not “tenable.”

Ukrainian officials, including President Volodymyr Zelenskyy, have pleaded for the MiG-29 transfer. And while they have support from a bipartisan group of U.S. lawmakers, the Biden administration, citing assessments from senior American military commanders in Europe, has said the additional aircraft would offer only minimal value to Ukraine given the contested nature of its airspace.

“We believe the most effective way to support the Ukrainian military in their fight against Russia is to provide increased amounts of antitank weapons and air-defense systems, which is ongoing with the international community,” said Air Force Gen. Tod Wolters, the chief of U.S. European Command. “The Ukrainians are making excellent use of these weapons now.”

NATO countries have been limited in what they can send to Ukraine due in part to the Ukrainian military’s training, which centers heavily on Soviet- or Russian-made weapons that exist in dwindling stock.

Ukraine has sought the S-300 surface-to-air missile system, said a senior European official familiar with the situation. There are ongoing talks about that possibility, the official said, but questions remain about whether there are any that can be spared.

“Nobody wants to mishandle expectations,” said the official, who spoke on the condition of anonymity.

Ukrainian troops also are trained on the Buk surface-to-air missile system. Some eastern European countries possess both systems, but in small numbers, the European official said.

Austin’s upcoming visit to Slovakia could be used to secure additional agreements to use its railways to facilitate faster transfers of humanitarian aid and fuel into Ukraine, the European official said. Unlike some other countries nearby, Slovakia has a shipping hub that operates on both European standard-gauge rail and the wider-gauge rail used in Ukraine.

Assessments of how long the Ukrainians can continue to hold out against the Russian onslaught are intertwined with what weapons they already have.

On Friday, a bipartisan group of lawmakers in the United States sent a letter to President Joe Biden asking him to transfer manned and unmanned aircraft to Ukraine, saying that “despite heroic and skilled resistance,” the Russians had achieved air superiority over the Ukrainians. U.S. defense officials noted the Russians have a significantly larger air force but had been unable to control the skies over Ukraine.

“Russia’s advantage in this domain could soon develop into air dominance if the Ukrainians do not receive necessary military aid,” the lawmakers said.

The lawmakers called for the continued delivery of Stinger missiles, man-portable anti-aircraft weapons, but also called for the United States to facilitate the transfer of S-300 systems.

“Providing Soviet-era platforms that Ukrainian servicemembers have previously operated and maintained will be essential to their success on the battlefield and will also protect U.S. defense technology from falling into the hands of the Russians,” lawmakers said in the letter.

They also urged the president to reconsider the Polish MiG decision and said the Biden administration should look for ways to send more armed drones and Su-25 aircraft to Ukraine. The jet is used by a handful of U.S. allies and partners, including Bulgaria, a NATO nation.

The lawmakers signing the letter Friday included numerous U.S. veterans.

Rep. Jason Crow, D-Colo., a former Army Ranger who is among the signatories, said the administration was drawing a nonsensical distinction between acceptable weapons and those it considers potentially too escalatory.

“I don’t believe there is a distinction between providing a MiG and providing a Javelin and a Stinger,” he said. “These are defensive systems; we are not providing offensive capabilities because Ukraine is not on the offense, all they’re doing is fighting for their survival and trying to maintain their democracy against a Vladimir Putin invasion.”

Crow also said that while the provision of Javelins and Stingers has been useful thus far, the West must step up its contributions to give Ukraine a chance to sustain its resistance — particularly in the skies, where Zelenskyy’s forces are trying to prevent Russia from achieving superiority.

“Short of MiGs, they need more advanced ground-to-air missile systems,” Crow said, noting that the Stingers provided to Ukraine work well against helicopters and planes conducting low-altitude bombing runs, but aren’t enough to ground the Russian air force. “They need something that can reach out and conduct regional air defense, higher altitude air defense.”

Rep. Michael Waltz, R-Fla., who has served in Special Forces units, said that transferring S-300 systems and components of them, such as radar, could be helpful.

Among Western weapons, the Ukrainians also could benefit from the Avenger system, which allows Stinger missiles to be mounted to vehicles, he said. Other American air-defense systems, including Patriot missiles, require too much training to make them practical in the near term.

The Ukrainians could benefit, too, from counter-battery radar systems, which scan for the originating locations of incoming artillery fire, and its equipment they’ve asked for before, Waltz said.

“At a minimum, once the Russians know you have that, they won’t be able to just sit there and lob round after round after round,” Waltz said. “It degrades their ability to just sit there and pummel you. They have to move, obviously, because now you have the ability to counter-battery.”

The Ukrainian military has 56 fighter jets left and is using them just five to 10 hours per day, a senior U.S. defense official said Friday. The official, speaking on the condition of anonymity, cited that as one reason the administration had decided against transferring the MiGs to Ukraine, noting that Russian surface-to-air missile systems can reach just about anywhere over the war zone.

U.S. officials continue to speak to the Ukrainians regularly, the defense official said, and the “things they need the most” are anti-aircraft and anti-armor weapons, such as Javelin missiles. For items the Ukrainians need that the United States does not have, the administration is working with allies and partners “who might have inventory.”

“We’re trying to help them further that defense in ways and with systems that they know how to use already,” the official said. “They’re good at it, and it’s having effect on the Russians.”

Information for this article was contributed by Ashley Parker of The Washington Post.